1. Field of the Invention
This invention relates to diode pumped Nd:YVO.sub.4 lasers, and more particularly to diode pumped Nd:YVO.sub.4 lasers with Nd doping levels of less than 0.5%.
2. Description of Related Art
The most common gain media used for diode pumped lasers is Nd:YAG and efficient systems can be constructed by end pumping with laser diodes and laser diode arrays. To build an efficient end pumped Nd:YAG laser, the pump light from the diode, which is typically not in a diffraction limited beam, must be focussed tightly into the gain media. To obtain TEM.sub.00 operation, which is desirable for many applications, the pump light must be focussed to a spot size smaller than the intracavity mode. In addition, since the pump light diverges more quickly than the intracavity mode, it must be absorbed in a short distance before it will diverge to a size larger than the intracavity mode. Thus tight focussing and short absorption depths were necessary to build efficient TEM.sub.00 Nd:YAG lasers pumped by diode lasers and diode arrays. These techniques are described in U.S. Pat. Nos. 4,635,056; 4,701,929; and 4,756,003.
The pump power available from these diode pump sources has increased steadily from 1 W diodes to 20 W diode bars and most recently to 40 W bars at 809 nm. As the pump power increased, several problems were encountered scaling the Nd:YAG lasers to higher power. For the YAG host in particular, increased pump power per unit area leads to increased birefringence. The gain media depolarizes the intra cavity beam; this leads to losses when polarized output is desired. A solution to the birefringence problem is to substitute Nd:YLF as the gain media. YLF is a birefringent material and naturally produces polarized output, even under high thermal loading. YLF, however, suffers from fracture problems as the pump power and hence the thermal loading is increased. An alternative material which is also naturally polarized and less susceptible to fracture is Nd:YVO.sub.4 (Nd:Vanadate or Vanadate).
As the pump power incident on the Vanadate crystal is increased, thermal lensing becomes the limiting factor. At high pump powers the lens becomes very strong with focal lengths as short as 10 cm. Although this strong lens can be largely compensated by clever cavity design, the aberrations in the lens eventually degrade the performance of the laser. Thus, in order to take advantage of the new higher power diode bars as pump sources, a solution to the aberrated thermal lens in Vanadate is needed.
The power of the lens in a diode pumped Vanadate laser is due to two major contributions: the lens due to the index change in the bulk and the lens due to the bulge in the surface of the crystal. One solution to reducing the surface bulge is to optically contact undoped Vanadate on the end of the gain media. These end caps do not reduce the lens in the bulk however, which is the subject of the following disclosure. Another technique to reduce the surface bulge is to pass the pump light through the crystal more than once. For example, a highly reflective coating for the pump light can be placed on the second surface of the crystal. The pump light will then pass twice through the crystal causing the pump to be absorbed more homogeneously throughout the crystal and causing less heating near the surface. Either of these techniques may be used in combination with the method described below to reduce the thermal lens even further.
There is a need for a Vanadate laser or laser system with higher powers. There is also a need for a Vanadate laser or laser system with a reduced lens in the bulk of the crystal.